In a wireless communication network a UE will typically communicate with other UEs via the network. For example, a signal from one UE will be transmitted to the network and then the network will in turn transmit that signal to the proper UE recipient. However, a UE can also communicate with other UE's directly by way of device-to-device (D2D) communication. Proximity Services (ProSe) may be one of the building blocks in the progress of the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) from Release 12 onwards. ProSe entails discovery of a UE within proximity of another UE, based on direct D2D radio signals between the UEs or network side proximity detection, and an optimized communication path for UEs in proximity to one another. This can be realized by the direct communication between UEs or a local switch within the Evolved Universal Terrestrial Radio Access Network (E-UTRAN, also referred to as a LTE network).
A UE which is located near a cell edge may want to participate in device-to-device (D2D) communications with another UE. This same UE may also be in a low duty cycle to conserve its limited (battery) power supply. Such a UE could receive discovery signals from that other UE, which may be in the first UE's own cell or in an adjacent neighbour cell. A problem that arises in this scenario is enabling the UEs to find one another, which becomes more difficult in the above low duty cycle example.